The invention concerns a method of searching for a track on a rotating disk-like data carrier with tracks that data can be written into and/or read out of by means of a read-and-write head that can be positioned over any track on the data carrier by means of a controlled coarse drive mechanism and of a controlled fine drive mechanism that is mechanically connected to the coarse drive mechanism.
Rotating data carriers that can be read with a reading head provided with an optical system for reading data are employed for example in CD players. The use of such rotating disk-like data carriers, which are read optically, is, however, in no way restricted to record players. Videorecorders with optically scanned disks are also available. In one case we speak of audio disks and in the other of video disks.
The data carrier--the CD (CD stands for compact disk)--rotates at a speed that is relatively higher than that of a conventional record player. The information--sound, picture signals, or both for example--which are stored on the CD along a spiral or in concentric circles, is read out by means of an optical system in the reading head, which usually operates with lasers. The individual windings of the spirals or the individual concentric circles constitute the tracks.
The article, "The Compact Disk Digital Audio System," by X. G. Carasso, J. B. H. Peek, and J. P. Sinjou in Philips Technical Review, Vol. 40, No. 6 (1982) describes the design of a CD player.
The reading head must be accurately guided from track to track in order to scan the data carrier--the CD. In order to read out any desired data, it must also be possible to position the reading head precisely at any desired track. The reading head is guided and positioned by means of a controlled coarse drive mechanism and of a controlled fine drive mechanism that is mechanically connected to the coarse drive mechanism. The coarse drive mechanism can for example be a spindle driven by a servomotor that carries out the coarse positioning, whereas the fine drive mechanism, which entrained as a result of the mechanical connection, assumes the function of precise positioning. The coarse drive mechanism moves radially in relation to the center--the point of rotation, that is--of the rotating disk. The fine drive mechanism is mechanically mounted and is deflected by means of electromagnets such that the scanned beam of light is radially diverted to a prescribed extent out of the vertical both toward and away from the point of rotation. This deflection results in fine adjustment at the desired track.
FIG. 3 illustrates the system. A CD is driven by a motor M. The reading head, often called the optical scanning system or optical pick-up, is coarsely positioned by means of a spindle SP. A servomotor SM, which is governed by the same controls RG as the coarse drive mechanism, drives spindle SP. The controls RF for the fine drive mechanism also govern electromagnets EM that divert a beam L of light out of the vertical.
When tracks are being searched for, tracks that are not too remote from one another can be located with the fine drive mechanism. When, however, data are to be read out of a track that is farther away, the coarse drive mechanism must be activated. To keep access times brief the coarse drive mechanism must be guided over the tracks at high speed.
EPA 0 090 379 describes a device with a coarse drive mechanism and a fine drive mechanism for reading a rotating data carrier. The application mentions that high speeds on the part of the coarse drive mechanism can as a result of the inertia that occurs lead to undesired mechanical oscillations on the part of the fine drive mechanism, so that the fine drive-mechanism control loop can no longer engage. The fine drive-mechanism control loop is accordingly interrupted while the coarse drive mechanism is in operation. An acceleration signal generated by a device that measures acceleration and speed and corresponding to the instantaneous motion of the coarse drive mechanism is simultaneously supplied to the fine drive mechanism. The acceleration signal attenuates the mechanical oscillations of the fine drive mechanism to such an extent that the fine drive-mechanism control loop can engage even when the coarse drive mechanism is in operation.
At the commencement of the process of searching for a particular track, which will from now on be called the target track herein, the number of [tracks] etween the track just scanned, which will from now on be called the departure track herein, and the target track is determined. The tracks traveled by the fine drive mechanism will now be counted during the search process.
Low speeds on the part of the coarse drive mechanism during the search for the target track, speeds that occur in particular while the coarse drive mechanism is accelerating and braking, necessitate not only longer access times but also result as will be described later herein in errors in searching for a track due to the eccentricity of the disk and of the disk-drive mechanism. The aforesaid European application, however, does not discuss these errors.
Because the eccentricity of the both disk itself and of the disk-drive mechanism is very great in comparison to the distance between two adjacent tracks--approximately 200 to 300 to 300 as against 1.6 .mu.m--one or more tracks that have already been traveled and accordingly counted can, when the coarse drive mechanism is operating at low speeds, be counted once or several times more because the coarse drive mechanism may travel over them two times or more in such cases. This will position the fine drive mechanism as it travels along with the coarse drive mechanism so far away from the target track that it will no longer be able to aim the beam of light at the target track.
One way of avoiding false positioning due to too low a speed on the part of the coarse drive mechanism is to establish what direction the coarse drive mechanism is actually travelling over the tracks in while the tracks are being counted by means of a special circuit called an orientation logical circuit. Since this solution, however, is expensive, it is reserved for the more costly equipment of the higher classes.
The coarse drive mechanism in the device disclosed in EPA 0 090 379 is uniformly accelerated until it arrives exactly between the departure track and the target track. Its speed accordingly increases linearly up to that point. Because the acceleration changes its mathematical sign while remaining constant in quantity at the midpoint between the departure track and the target track, the coarse drive mechanism is uniformly decelerated, and its speed will decrease linearly starting from the midpoint between the departure track and the target track and finally becoming zero when the coarse drive mechanism arrives at the target track. At the beginning and end of the path traveled by the coarse drive mechanism, in the vicinity of both the departure track and the target track. that is, its speed can be so low that the aforesaid errors due to the eccentricity of the disk and disk-drive mechanism can occur. To make it possible to reliably eliminate these errors the device disclosed in EPA 0 090 379 necessitates what is called an orientation logical circuit, which determines the direction that a track on the data carrier is actually being traveled over in. The orientation logical circuit and the device for measuring speed and acceleration increase the cost and accordingly make the equipment provided with the device disclosed in the European application more expensive.